Japan size Inner Diameter (mm) Inner Circumference (mm)
1 13.0 40.8
1.5 13.2 41.4
2 13.4 41.9
2.5 13.5 42.4
3 13.7 42.9
3.5 13.8 43.5
4 14.0 44.0
4.5 14.2 44.5
5 14.4 45.0
5.5 14.6 45.6
6 14.7 46.1
6.5 14.9 46.6
7 15.0 47.1
7.5 15.2 47.7
8 15.4 48.2
8.5 15.6 48.7
9 15.7 49.2
9.5 15.9 49.8
10 16.0 50.3
10.5 16.2 50.8
11 16.4 51.3
11.5 16.6 51.9
12 16.7 52.4
12.5 16.9 52.9
13 17.0 53.4
13.5 17.2 54.0
14 17.4 54.5
14.5 17.6 55.0
15 17.7 55.5
15.5 17.9 56.0
16 18.0 56.5
16.5 18.2 57.1
17 18.4 57.6
17.5 18.6 58.1
18 18.7 58.6
18.5 18.9 59.2
19 19.0 59.7
19.5 19.2 60.2
20 19.4 60.7
20.5 19.6 61.3
21 19.7 61.8
21.5 19.9 62.3
22 20.0 62.8
22.5 20.2 63.4
23 20.4 63.9
23.5 20.6 64.4
24 20.7 64.8
24.5 20.9 65.4
25 21.0 66.0

*The above diameter and circumference are approximate.